The present invention is directed to a system and method for controlling an electro-hydraulic valve arrangement. In particular the present invention is directed to a system and method for controlling an electro-hydraulic valve arrangement to provide make-up fluid to a hydraulic actuator.
Hydraulic actuators, such as piston/cylinder arrangements or fluid motors, are commonly used to move work implements, such as, for example, buckets or shovels. Each hydraulic actuator typically includes at least two fluid chambers that are disposed on opposite sides of a moveable element. The moveable element is, in turn, connected to the work implement that is to be moved. A pump is typically connected to the hydraulic actuator to selectively provide pressurized fluid to one or the other of the fluid chambers of the hydraulic actuator. These systems typically include an electro-hydraulic valve arrangement that selectively connects the pump with one of the fluid chambers.
When it is desirable to move the work implement in a certain direction, the electro-hydraulic valve arrangement is moved so that pressurized fluid is provided to one chamber of the hydraulic actuator at the same time that fluid is allowed to flow out of the other chamber. This creates a pressure differential over the moveable element of the hydraulic actuator. Provided that the force exerted on the moveable element by the pressurized fluid is great enough to overcome the resistant force of the work implement, the moveable element will move towards the area of lower fluid pressure existing in the opposite chamber of the hydraulic actuator, thereby moving the work implement.
An operator is typically provided with a control lever that governs the motion of the work implement. When the operator moves the control lever towards a first operative position, the electro-hydraulic valve arrangement is moved to allow pressurized fluid to flow into the first chamber of the hydraulic actuator and out of the second chamber, which results in the work implement moving in the first direction. Similarly, when the operator moves the control lever to a second operative, the electro-hydraulic valve arrangement is moved to allow pressurized fluid to flow into the second chamber of the hydraulic actuator and out of the first chamber, which results in the work implement moving in the second direction.
When the operator moves the lever to a neutral position, the electro-hydraulic valve arrangement closes so that fluid stops flowing to either side of the hydraulic actuator. If the operator abruptly moves the control lever to the neutral position, the momentum of the work implement will continue to act on the hydraulic actuator. If the work implement is carrying a heavy load, this momentum may increase the pressure in the hydraulic actuator, or a connecting fluid line, to a high level. A relief valve, disposed in the fluid line, may open to release fluid and reduce the pressure in the system.
The release of fluid from one chamber allows the moveable element to continue moving, thereby increasing the volume of the opposite chamber. If no additional fluid enters the opposite chamber, the pressure within the opposite chamber will drop. If the pressure drops enough, the hydraulic actuator may experience cavitation, which can be damaging to the equipment within the system. To prevent cavitation, additional fluid, or make-up fluid, must be added to the opposite chamber to compensate for the expulsion of fluid through the relief valve.
Typically, as shown in U.S. Pat. No. 5,921,165, additional valves are included in the hydraulic actuator control system to provide make-up fluid to the hydraulic actuator. These valves will open to provide the additional fluid to the hydraulic actuator when one of the chambers is susceptible to cavitation, i.e., experiencing a low or negative pressure. However, these types of arrangements are also costly in that additional valves and control devices are required. In addition, these types of arrangements provide very little control over when additional fluid is added to the system. For example, these types of arrangements do not provide appropriate make-up flow when all levers are in their neutral positions and a cylinder is still in motion.
The present invention provides a system and method for providing make-up fluid to a hydraulic actuator that solves all or some of the problems set forth above.
Accordingly, the present invention is directed to a method and system for controlling an electro-hydraulic valve arrangement. This method and system controls the electro-hydraulic valve arrangement, based on sensed parameters, to provide make-up fluid to a hydraulic actuator. The advantages and purposes of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purposes of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims.
To attain the advantages and in accordance with the purposes of the invention, as embodied and broadly described herein, the invention is directed to a method of controlling an electro-hydraulic valve arrangement that is actuated by a control lever having a neutral position and is disposed between a pump having a stand-by pressure and an actuator. According to the method, an actuator pressure that is representative of the fluid pressure in the actuator is sensed. The actuator pressure is compared to the pump stand-by pressure. The control of the control lever is overridden to allow fluid to flow from the pump through the electro-hydraulic valve arrangement to the actuator when the difference between the pump stand-by pressure and the actuator pressure is greater than a predetermined limit.
In another aspect, the invention is directed to a system for controlling a flow of fluid to a hydraulic actuator. The system includes a pump that has a stand-by pressure. An electro-hydraulic valve arrangement is in fluid connection with the pump and the hydraulic actuator. The electro-hydraulic valve controls the amount of fluid flowing from the pump to the hydraulic actuator. A control lever having a neutral position is provided. Movement of the control lever to the neutral position acts to close the electro-hydraulic valve arrangement and prevent the flow of fluid to the hydraulic actuator. A pressure sensor senses a first pressure representative of the pressure of the fluid within the hydraulic actuator. A control device is provided to override the closing of the electro-hydraulic valve arrangement when the difference between the pump stand-by pressure and the first pressure is greater than a predetermined limit.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.